Wireless power feeder and wireless power transmission system

ABSTRACT

Power is fed from a feeding coil L 2  to a receiving coil L 3  by magnetic resonance. An oscillator  202  alternately turns ON/OFF switching transistors Q 1  and Q 2  to cause AC current IS of drive frequency fo to flow in a transformer T 2  primary coil Lb. The AC current IS causes AC current I 1  to flow in an exciting coil L 1  and causes AC current I 2  to flow in the feeding coil L 2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless power feeder for feedingpower by wireless and a wireless power transmission system.

2. Description of Related Art

A wireless power feeding technique of feeding power without a power cordis now attracting attention. The current wireless power feedingtechnique is roughly divided into three: (A) type utilizingelectromagnetic induction (for short range); (B) type utilizing radiowave (for long range); and (C) type utilizing resonance phenomenon ofmagnetic field (for intermediate range).

The type (A) utilizing electromagnetic induction has generally beenemployed in familiar home appliances such as an electric shaver;however, it can be effective only in a short range of severalcentimeters. The type (B) utilizing radio wave is available in a longrange; however, it cannot feed big electric power. The type (C)utilizing resonance phenomenon is a comparatively new technique and isof particular interest because of its high power transmission efficiencyeven in an intermediate range of about several meters. For example, aplan is being studied in which a receiving coil is buried in a lowerportion of an EV (Electric Vehicle) so as to feed power from a feedingcoil in the ground in a non-contact manner. The wireless configurationallows a completely insulated system to be achieved, which is especiallyeffective for power feeding in the rain. Hereinafter, the type (C) isreferred to as “magnetic field resonance type”.

The magnetic field resonance type is based on a theory published byMassachusetts Institute of Technology in 2006 (refer to Patent Document1). In Patent Document 1, four coils are prepared. The four coils arereferred to as “exciting coil”, “feeding coil”, “receiving coil”, and“loading coil” in the order starting from the feeding side. The excitingcoil and feeding coil closely face each other for electromagneticcoupling. Similarly, the receiving coil and loading coil closely faceeach other for electromagnetic coupling. The distance (intermediatedistance) between the feeding coil and receiving coil is larger than thedistance between the exciting coil and feeding coil and distance betweenthe receiving coil and loading coil. This system aims to feed power fromthe feeding coil to receiving coil.

When AC power is fed to the exciting coil, current also flows in thefeeding coil according to the principle of electromagnetic induction.When the feeding coil generates a magnetic field to cause the feedingcoil and receiving coil to magnetically resonate, large current flows inthe receiving coil. At this time, current also flows in the loading coilaccording to the principle of electromagnetic induction, and power istaken out from a load R connected in series to the loading coil. Byutilizing the magnetic field resonance phenomenon, high powertransmission efficiency can be achieved even if the feeding coil andreceiving coil are largely spaced from each other.

CITATION LIST Patent Document

-   [Patent Document 1] U.S. Pat. Appln. Publication No. 2008/0278264-   [Patent Document 2] Jpn. Pat. Appln. Laid-Open Publication No.    2006-230032-   [Patent Document 3] International Publication Pamphlet No.    WO2006/022365-   [Patent Document 4] U.S. Pat. Appln. Publication No. 2009/0072629

In a circuit of Patent Document 2 (FIG. 1), when MOSFETs Q1 and Q4 areON state and MOSFETs Q2 and Q3 are OFF state, voltage of DC power supplyV1 is applied to a series circuit composed of four elements: MOSFET Q1,primary coil L1, capacitor C1, and MOSFET Q4. A series circuit of theprimary coil L1 and capacitor C1 constitutes a primary side seriesresonance circuit (refer to paragraph [0030] of Patent Document 2), sothat the impedance thereof is low. As a result, most of the voltage ofthe DC power supply V1 is applied to two MOSFETs. Application ofexcessively high voltage to the MOSFET may destroy the MOSFET, so thatit is not appropriate to use a high voltage supply as the DC powersupply V1. In the case of Patent Document 2, the power supply voltage ofthe DC power supply V1 is set to 5 (V).

In the case where a commercial power supply is used as an input powersupply for wireless power feeding, it is necessary to prevent highvoltage from being applied to a switching element such as MOSFET. Therecan be considered a method of using an AC adapter to reduce a commercialpower supply voltage as one method for preventing high voltage frombeing applied to a switching element. However, the use of the AC adapteris undesirable because it may cause power loss or an increase in cost.

Further, the commercial power supply voltage differs from one country orregion to another (For example, 100 (V) in Japan and Taiwan, 120 (V) inUnited States, 110 (V) or 220 (V) in China, and 115 (V) to 240 (V) inIndia). Thus, a system capable of meeting the difference in thecommercial power supply voltage is desirable.

SUMMARY

The present invention has been made based on the above problems, and amain object thereof is to achieve wireless power feeding of a magneticfield resonance type capable of being operated safely even in the casewhere high voltage is input.

A wireless power feeder according to a first aspect of the presentinvention feeds power from a feeding coil to a receiving coil bywireless using a magnetic field resonance phenomenon between the feedingcoil and receiving coil. The wireless power feeder includes a powertransmission control circuit, a feeding coil, and an exciting circuit.The power transmission control circuit makes first and second switchesconnected in series respectively to first and second current pathsalternately conductive to feed AC power to a first coil included in boththe first and second current paths. The exciting circuit is a circuit inwhich a second coil magnetically coupled to the first coil and anexciting coil magnetically coupled to the feeding coil are connected.The exciting circuit receives AC power from the power transmissioncontrol circuit through the second coil and feeds the AC power to thefeeding coil through the exciting coil.

A wireless power feeder according to a second aspect of the presentinvention feeds power from a feeding coil to a receiving coil bywireless using a magnetic field resonance phenomenon between the feedingcoil and receiving coil. The wireless power feeder includes: a powertransmission control circuit that includes first and second currentpaths and makes first and second switches connected in seriesrespectively to the first and second current paths alternatelyconductive to feed AC power to a first coil included in both the firstand second current paths; and a feeding coil circuit in which thefeeding coil and a second coil are connected. The feeding coil circuitreceives AC power from the power transmission control circuit throughthe second coil magnetically coupled to the first coil.

By making current to alternately flow in the first coilbi-directionally, AC power can be fed from the first coil of the powertransmission control circuit to second coil of the exciting circuit orfeeding coil circuit. This configuration eliminates the need to providea resonance circuit in the power transmission control circuit itself,thus eliminating the need to connect a capacitor in series to the firstcoil. This makes it easy to apply high voltage to the first coil,facilitating suppression of voltage to be applied to a switch such as aMOSFET. Further, by adjusting the ratio of the number of windingsbetween the first and second coils, the magnitude of voltage to be fedto the second coil can be adjusted.

The power transmission control circuit may include: a bridgerectification circuit in which diodes are bridge-connected at four firstto fourth connection points; an AC power supply connected between thefirst and second connection points of the bridge rectification circuit;and a capacitor connected between the third and fourth connection pointsof the bridge rectification circuit and charged by the AC power supplyand feed AC power to the first coil using the capacitor as a DC voltagesource.

Rectifying AC voltage in the rectification circuit allows the capacitorto be charged by an AC power supply. By using the capacitor as a DCvoltage source, it is possible to make the most of the power supplyvoltage of the AC power supply. This facilitates setting of high inputvoltage.

Two capacitors may be connected in series between the third and fourthconnection points, and midpoint of the two capacitors may be connectedto one of the first and second connection points. In this case, the twocapacitors can easily be charged by the maximum voltage value of the ACpower supply, allowing the input voltage to be set much higher. Further,a switch for controlling electrical conduction of a path extending fromthe midpoint to one of the first and second connection points may beprovided.

The number of windings of the first coil may be greater than that of thesecond coil. In this case, the value of the voltage to be fed to theexciting circuit and the like can be reduced to an appropriate leveleven if the input voltage is high.

The input voltage of the power transmission control circuit may beapplied to one of a series circuit composed of the first coil and firstswitch and a series circuit composed of the first coil and secondswitch. Setting the inductance of the first coil to a large value easilyprevents excessive voltage from being applied to the first or secondswitch.

The power transmission control circuit may make the feeding coil thatdoes not substantially resonate with circuit elements on the powerfeeding side feed the AC power to the receiving coil. The “substantiallydoes not resonate” mentioned here means that the resonance of thefeeding coil is not essential for the wireless power feeding, but doesnot mean that even an accidental resonance of the feeding coil with somecircuit element is eliminated. A configuration may be possible in whichthe feeding coil does not form, together with power feeding side circuitelements, a resonance circuit that resonates with at a resonance pointcorresponding to the resonance frequency of the receiving coil. Further,a configuration may be possible in which no capacitor is inserted inseries or in parallel to the feeding coil.

The feeding coil may be connected to a capacitor and form a circuitwhich resonates at a resonance frequency of the receiving coil.

A wireless power transmission system according to the present inventionincludes: the wireless power feeder described above, a receiving coil;and a loading coil that is magnetically coupled to the receiving coiland receives power that the receiving coil has received from a feedingcoil.

The receiving coil may be connected to a capacitor and form a circuitwhich resonates at a resonance frequency of the feeding coil.

It is to be noted that any arbitrary combination of the above-describedstructural components and expressions changed between a method, anapparatus, a system, etc. are all effective as and encompassed by thepresent embodiments.

According to the present invention, even in the case where high voltageis input, wireless power feeding of a magnetic field resonance type canbe performed safely.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following description of certain preferred embodimenttaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a principle view of a wireless power transmission systemaccording to a first embodiment of the present invention;

FIG. 2 is a configuration view of a wireless power transmission systemaccording to a first embodiment of the present invention;

FIG. 3 is a time chart illustrating the voltage/current changingprocess;

FIG. 4 is a graph illustrating a relationship between the number ofwindings of the secondary coil in the coupling transformer T2 andsecondary voltage;

FIG. 5 is configuration view of a modification of the wireless powertransmission system;

FIG. 6 is a principle view of a wireless power transmission systemaccording to a second embodiment of the present invention; and

FIG. 7 is a system configuration view of a wireless power transmissionsystem according to a second embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENT

A preferred embodiment of the present invention will be explained belowin detail with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a view illustrating operation principle of a wireless powertransmission system 100 according to the first embodiment. The wirelesspower transmission system 100 in the first embodiment includes awireless power feeder 116 and a wireless power receiver 118. Thewireless power feeder 116 includes a power feeding LC resonance circuit300. The wireless power receiver 118 includes a receiving coil circuit130 and a loading circuit 140. A power receiving LC resonance circuit302 is formed by the receiving coil circuit 130.

The power feeding LC resonance circuit 300 includes a capacitor C2 and afeeding coil L2. The power receiving LC resonance circuit 302 includes acapacitor C3 and a receiving coil L3. The values of the capacitor C2,feeding coil L2, capacitor C3, and receiving coil L3 are set such thatthe resonance frequencies of the feeding LC resonance circuit 300 andreceiving LC resonance circuit 302 coincide with each other in a statewhere the feeding coil L2 and receiving coil L3 are disposed away fromeach other far enough to ignore the magnetic field couplingtherebetween. This common resonance frequency is assumed to be fr0.

In a state where the feeding coil L2 and receiving coil L3 are broughtclose to each other in such a degree that they can bemagnetic-field-coupled to each other, a new resonance circuit is formedby the power feeding LC resonance circuit 300, power receiving LCresonance circuit 302, and mutual inductance generated between them. Thenew resonance circuit has two resonance frequencies fr1 and fr2(fr1<fr0<fr2) due to the influence of the mutual inductance. When thewireless power feeder 116 supplies AC power from a power feeding sourceVG to the power feeding LC resonance circuit 300 at the resonancefrequency fr1, the power feeding LC resonance circuit 300 constitutingapart of the new resonance circuit resonates at a resonance point 1(resonance frequency fr1). When the power feeding LC resonance circuit300 resonates, the feeding coil L2 generates an AC magnetic field of theresonance frequency fr1. The power receiving LC resonance circuit 302constituting apart of the new resonance circuit also resonates byreceiving the AC magnetic field. When the power feeding LC resonancecircuit 300 and power receiving LC resonance circuit 302 resonate at thesame resonance frequency fr1, wireless power feeding from the feedingcoil L2 to receiving coil L3 is performed with the maximum powertransmission efficiency. Received power is taken from a load LD of thewireless power receiver 118 as output power. Note that the new resonancecircuit can resonate not only at the resonance point 1 (resonancefrequency fr1) but also at a resonance point 2 (resonance frequencyfr2).

The wireless power feeder 116 based on this principle does not includean exciting coil. In case of including the exciting coil L1, theprinciple of a wireless feeding is basically the same. It will bedescribed later related with FIG. 10 and the like in case of notincluding the exciting coil L1.

FIG. 2 is a system configuration view of a wireless power transmissionsystem 100 according to the first embodiment. The wireless powertransmission system 100 includes a wireless power feeder 116 and awireless power receiver 118. The wireless power feeder 116 includes, asbasic components, a power transmission control circuit 200, an excitingcircuit 110, and a feeding coil circuit 120. The wireless power receiver118 includes a receiving coil circuit 130, and a loading circuit 140.

A distance of several meters is provided between a feeding coil L2 ofthe feeding coil circuit 120 and a receiving coil L3 of the receivingcoil circuit 130. The wireless power transmission system 100 mainly aimsto feed AC power from the feeding coil L2 to receiving coil L3 bywireless. The wireless power transmission system according to thepresent embodiment is assumed to operate at a resonance frequency fr1 of100 kHz or less. In the present embodiment, a resonance frequency fr1 isset to 50 kHz. Note that the wireless power transmission systemaccording to the present embodiment can operate in a high-frequency bandsuch as ISM (Industry-Science-Medical) frequency band.

In the exciting circuit 110, an exciting coil L1 and a transformer T2secondary coil L1 are connected in series. The transformer T2 secondarycoil L1 constitutes a coupling transformer T2 together with atransformer T2 primary coil Lb and receives AC power from the powertransmission control circuit 200 by electromagnetic induction. Thenumber of windings of the exciting coil L1 is 1, cross-sectional shapeof a coil conductor thereof is a rectangle of 0.6 mm×6.0 mm, and shapeof the exciting coil L1 itself is a square of 210 mm×210 mm. In FIG. 2,the exciting coil L1 is represented by a circle for simplicity. Othercoils are also represented by circles for the same reason. All the coilsillustrated in FIG. 2 are made of copper. Current I1 flowing in theexciting circuit 110 is AC. The direction of an arrow in the diagram ofthe exciting circuit 110 indicates the positive direction, and directionopposite to the direction of the arrow indicates the negative direction.

In the feeding coil circuit 120, a feeding coil L2 and a capacitor C2are connected in series. The exciting coil L1 and feeding coil L2 faceeach other. The distance between the exciting coil L1 and feeding coilL2 is as comparatively small as 10 mm or less. Thus, the exciting coilL1 and feeding coil L2 are electromagnetically strongly coupled to eachother. The number of windings of the feeding coil L2 is 7,cross-sectional shape of a coil conductor thereof is a rectangle of 0.6mm×6.0 mm, and shape of the feeding coil L2 itself is a square of 280mm×280 mm. When the AC current I1 is made to flow in the exciting coilL1, an electromotive force occurs in the feeding coil L2 according tothe principle of electromagnetic induction to cause AC current I2 toflow in the feeding coil circuit 120. The direction of an arrow in thediagram of the feeding coil circuit 120 indicates the positivedirection, and direction opposite to the direction of the arrowindicates the negative direction. The flowing directions of the currentI2 and current I1 are the same (in-phase). The AC current I2 isconsiderably larger than the AC current I1. The values of the feedingcoil L2 and capacitor C2 are set such that the resonance frequency fr1is 50 kHz.

In the receiving coil circuit 130, a receiving coil L3 and a capacitorC3 are connected in series. The feeding coil L2 and receiving coil L3face each other. The distance between the feeding coil L2 and receivingcoil L3 is as comparatively large as about 0.2 m to 1 m. The number ofwindings of the receiving coil L3 is 7, cross-sectional shape of a coilconductor thereof is a rectangle of 0.6 mm×6.0 mm, and shape of thereceiving coil L3 itself is a square of 280 mm×280 mm. The values of thereceiving coil L3 and capacitor C3 are set such that the resonancefrequency fr1 is also 50 kHz. Thus, the feeding coil L2 and receivingcoil L3 need not have the same shape. When the feeding coil L2 generatesa magnetic field at the resonance frequency fr1, the feeding coil L2 andreceiving coil L3 magnetically resonate, causing large current I3 toflow in the receiving coil circuit 130. The direction of an arrow in thediagram of the receiving coil circuit 130 indicates the positivedirection, and direction opposite to the direction of the arrowindicates the negative direction. The flowing directions of the currentI2 and current I3 are the same (in-phase).

In the loading circuit 140, a loading coil L4 and a load LD areconnected in series. The receiving coil L3 and loading coil L4 face eachother. The distance between the receiving coil L3 and loading coil L4 isas comparatively small as about 10 mm or less. Thus, the receiving coilL3 and loading coil L4 are electromagnetically strongly coupled to eachother. The number of windings of the loading coil L4 is 1,cross-sectional shape of a coil conductor thereof is a rectangle of 0.6mm×6.0 mm, and shape of the loading coil L4 itself is a square of 300mm×300 mm. When the current I3 is made to flow in the receiving coil L3,an electromotive force occurs in the loading circuit 140 to causecurrent I4 to flow in the loading circuit 140. The direction of an arrowin the diagram of the loading circuit 140 indicates the positivedirection, and direction opposite to the direction of the arrowindicates the negative direction. The flowing directions of the currentI3 and current I4 are the same (in-phase). That is, the current I2 andcurrent I4 are also in-phase. The AC power fed from the feeding coil L2of the wireless power feeder 116 is received by the receiving coil L3 ofthe wireless power receiver 118 and taken from the load LD.

If the load LD is connected in series to the receiving coil circuit 130,the Q-value of the receiving coil circuit 130 is degraded. Therefore,the receiving coil circuit 130 for power reception and loading circuit140 for power extraction are separated from each other. In order toenhance the power transmission efficiency, the center lines of theexciting coil L1, the feeding coil L2, receiving coil L3, and loadingcoil L4 are preferably made to coincide with one another.

A configuration of the power transmission control circuit 200 will bedescribed. The power transmission control circuit 200 is a half-bridgetype electric circuit and roughly includes a power supply section 102and a control section 104. In the power supply section 102, a capacitorC5 is connected between points A and C of FIG. 2, and a capacitor C6 isconnected between points C and B of FIG. 2. These capacitors are chargedby the voltage of an AC power supply 106. The voltage (voltage betweenpoints A and C) of the capacitor C5 is referred to as VA, voltage(voltage between points C and B) of the capacitor C6 is referred to asVB, and VA+VB (voltage between points A and B) is referred to as inputvoltage Vin.

The AC power supply 106 is a usual commercial power supply. The ACvoltage of the AC power supply 106 is rectified by a rectificationcircuit 108 to be converted into DC voltage. The rectification circuit108 is a circuit in which four diodes D1 to D4 are bridge-connected toone another. One end of the AC power supply 106 is connected to aconnection point P1 between the diodes D1 and D2, and the other endthereof is connected to a connection point P2 between the diodes D3 andD4. A connection point P3 between the diodes D1 and D3 is connected topoint B, a negative-side of the capacitor C6, and a connection point P4between the diodes D2 and D4 is connected to point A, a positive-side ofthe capacitor C5. The connection point P2 is connected a point C througha switch SW. As a modification, in place of the connection point P2,connection point P1 may be connected to the point C.

Current flowing from the AC power supply 106 is referred to as “currentIR”. The current IR is AC current. The direction of an arrow in thediagram of the power supply section 102 indicates the positivedirection, and direction opposite to the direction of the arrowindicates the negative direction.

When the switch SW is turned ON, electrical conduction is made betweenthe negative electrode of the capacitor C5 and connection point P2 andbetween the positive electrode of the capacitor C6 and connection pointP2. When flowing in the positive direction, the current IR flows fromthe AC power supply 106, passes through the connection point P1, diodeD2, connection point P4, point A, capacitor C5, point C, and switch SWin this order, and returns to the AC power supply 106. When flowing inthe negative direction, the current IR flows from the AC power supply106, passes through the switch SW, point C, capacitor C6, point B,connection point P3, diode D1, and connection point P1 in this order,and returns to the AC power supply 106. Direction of the current IRchanges at the frequency of the AC power supply 106, for example, 50kHz.

As described above, when the switch SW is turned ON, the positivevoltage of the AC power supply 106 is applied only to the capacitor C5,and negative voltage thereof is applied only to the capacitor C6. Forexample, when the effective voltage of the AC power supply 106 is 100(V), about 141 (V) which is the maximum voltage value is applied to thecapacitors C5 and C6, respectively. As a result, input voltage Vinbecomes 282 (V) (=141+141). That is, the capacitors C5 and C6 eachbecome a DC power supply of 141 (V).

When the switch SW is turned OFF, the path of the current IR is changed.When flowing in the positive direction, the current IR flows from the ACpower supply 106, passes through the connection point P1, diode D2,connection point P4, point A, capacitor C5, point C, capacitor C6, pointB, connection point P3, diode D3 and connection point P2 in this order,and returns to the AC power supply 106. When flowing in the negativedirection, the current IR flows from the AC power supply 106, passesthrough the connection point P2, diode D4, connection point P4, point A,capacitor C5, point C, capacitor C6, point B, connection point P3, diodeD1, and connection point P1, and returns to the AC power supply 106.

As described above, when the switch SW is turned OFF, both the positiveand negative voltages of the AC power supply 106 are applied betweenpoints A and B. For example, the effective voltage of the AC powersupply 106 is 200 (V), about 282 (V) which is the maximum voltage valueis applied to a series circuit of the capacitors C5 and C6. That is, theinput voltage Vin becomes 282 (V). The capacitors C5 and C6 each becomea DC power supply of 141 (V).

Irrespective of whether the effective voltage of the AC power supply 106is 100 (V) or 200 (V), the input voltage Vin can be kept at 284 (V) bycontrolling the switch SW. However, it is not always possible to keepthe input voltage Vin at a fixed value by controlling ON/OFF of theswitch SW. For example, when the effective voltage of the AC powersupply 106 is 100 (V) and 120 (V), the input voltage Vin becomesdifferent values. Even in this case, by adjusting the ratio of thenumber of windings of the coupling transformer T2 as described later,the magnitude of the voltage to be fed to the exciting circuit 110 canarbitrarily be set.

As illustrated in FIG. 2, the control section 104 has an electriccircuit having a vertically symmetrical shape. In the control section104, an oscillator 202 is connected to the primary side of a gate-drivetransformer T1. The oscillator 202 generates AC voltage Vo of a drivefrequency fo. Although the waveform of the AC voltage Vo may be a sinewave, it is assumed here that the voltage waveform is a rectangularwave. The drive frequency fo is generally 50 kHz which is equal to theresonance frequency fr1 and significantly higher than the frequency ofthe AC power supply 106. The voltage of the oscillator 202 is generallysignificantly lower than the voltage of the AC power supply 106.

The AC voltage Vo causes current of the drive frequency fo to flow in atransformer T1 primary coil Lh alternately in both positive and negativedirections. The transformer T1 primary coil Lh, transformer T1 secondarycoil Lf, and transformer T1 secondary coil Lg constitute a gate-drivecoupling transformer T1. Electromagnetic induction causes current toflow also in the transformer T1 secondary coil Lf and the transformer T1secondary coil Lg alternately in both positive and negative directions.

One end of the transformer T1 secondary coil Lf is connected to the gateof a switching transistor Q1, and the other end of the transformer T1secondary coil Lf is connected to the source of a switching transistorQ1. One end of the transformer T1 secondary coil Lg is connected to thegate of a switching transistor Q2, and the other end of the transformerT1 secondary coil Lg is connected to the source of a switchingtransistor Q2. When the oscillator 202 generates AC voltage Vo of thedrive frequency fo, voltage Vx (Vx>0) of the drive frequency fo isapplied alternately to the gates of the switching transistors Q1 and Q2.As a result, the switching transistors Q1 and Q2 are alternately turnedon/off at the drive frequency fo. The switching transistors Q1 and Q2are enhancement type MOSFET (Metal Oxide Semiconductor Field effecttransistor) having the same characteristics but may be other transistorssuch as a bipolar transistor. Further, other switches such as a relayswitch may be used in place of the transistor.

The drain of the switching transistor Q1 is connected to the positiveelectrode of the capacitor C5. The negative electrode of the capacitorC5 is connected to the source of the switching transistor Q1 through thetransformer T2 primary coil Lb. The potential of the negative electrodeof the capacitor C5 is assumed to be ground potential. The source of theswitching transistor Q2 is connected to the negative electrode of thecapacitor C6. The positive electrode of the capacitor C6 is connected tothe drain of the switching transistor Q2 through the transformer T2primary coil Lb. The potential of the positive electrode of thecapacitor C6 is ground potential.

Voltage between the source and drain of the switching transistor Q1 isreferred to as source-drain voltage VDS1, and voltage between the sourceand drain of the switching transistor Q2 is referred to as source-drainvoltage VDS2. Current flowing between the source and drain of theswitching transistor Q1 is referred to as source-drain current IDS1, andcurrent flowing between the source and drain of the switching transistorQ2 is referred to as source-drain current IDS2. The directions of arrowsin the diagram indicate the positive directions, and directions oppositeto the directions of the arrows indicate the negative directions.

When the switching transistor Q1 is turned conductive (ON), theswitching transistor Q2 is turned non-conductive (OFF). A main currentpath (hereinafter, referred to as “first current path”) at this timeextends from the positive electrode of the capacitor C5, passes throughthe point A, switching transistor Q1, transformer T2 primary coil Lb,and point C in this order, and returns to the negative electrode of thecapacitor C5. The switching transistor Q1 functions as a switch forcontrolling conduction/non-conduction of the first current path.

When the switching transistor Q2 is turned conductive (ON), theswitching transistor Q1 is turned non-conductive (OFF). A main currentpath (hereinafter, referred to as “second current path”) at this timeextends from the positive electrode of the capacitor C6, passes throughthe point C, transformer T2 primary coil Lb, switching transistor Q2,and point B in this order, and returns to the negative electrode of thecapacitor C6. The switching transistor Q2 functions as a switch forcontrolling conduction/non-conduction of the second current path.

Current flowing in the transformer T2 primary coil Lb in the powertransmission control circuit 200 is referred to as “current IS”. Thecurrent IS is AC current, and the current flow in the first current pathis defined as the positive direction and current flow in the secondcurrent path is defined as the negative direction.

When the oscillator 202 feeds the AC voltage Vo at the drive frequencyfo equal to the resonance frequency fr1, the first current path andsecond current path are alternately switched at the resonance frequencyfr1. Since the AC current IS of the resonance frequency fr1 flows in thetransformer T2 primary coil Lb, the AC current I1 flows in the excitingcircuit 110 at the resonance frequency fr1, and the AC current I2 of theresonance frequency fr1 flows in the feeding coil circuit 120. Thus, thefeeding coil L2 of the feeding coil circuit 120 and capacitor C2 are ina resonance state. The receiving coil circuit 130 is also a resonancecircuit of the resonance frequency fr1, so that the feeding coil L2 andreceiving coil L3 magnetically resonate. At this time, the maximumtransmission efficiency can be obtained.

In the first current path, the capacitor C5 functions as a DC voltagesource. Assuming that the both-end voltage of the transformer T2 primarycoil Lb is V1, voltage VA (=Vin/2) of the capacitor C5 is equal toVDS1+V1. The transformer T2 primary coil Lb need not LC-resonate withanother capacitor, so that a large value can be set as the inductancethereof. Thus, even in the case where the input voltage Vin is high,VDS1 becomes small when the inductance of the transformer T2 primarycoil Lb is large enough, which prevents excessive voltage from beingapplied between the source and drain of the switching transistor Q1.

In the second current path, the capacitor C6 functions as a DC voltagesource. The voltage VB of the capacitor C6 is equal to VDS2+V1. Also inthis case, when the inductance of the transformer T2 primary coil Lb islarge enough, the switching transistor Q2 is hardly destroyed.

That is, although the input voltage Vin becomes high when the powersupply voltage of the AC power supply 106 is high, most of the inputvoltage Vin is applied to the transformer T2 primary coil Lb, therebypreventing excessive voltage from being applied to the switchingtransistors Q1 and Q2. This is because that it is not necessary to makethe transformer T2 primary coil Lb LC-resonate in the power transmissioncontrol circuit 200.

Assume that the number of windings of the transformer T2 primary coil Lbis N1 and number of windings of the transformer T2 secondary coil Li isN2. The AC magnetic field generated by the AC current IS of thetransformer T2 primary coil Lb causes inductive current I1 having thesame phase as that of the AC current IS to flow in the transformer T2secondary coil Li. The coupling transformer T2 is, e.g., a smalltransformer (toroidal, EE-type, EI-type, etc.) using a ferrite core. Themagnitude of the current I1 is IS·(N1/N2) according to the law of equalampere-turn. The relationship between the voltage V2 of the transformerT2 secondary coil Li and voltage V1 of the transformer T2 primary coilLb is represented by V2=V1·(N2/N1). In the present embodiment, N1>N2 isestablished. Therefore, the voltage V1 is reduced to voltage V2 by thecoupling transformer T2. By adjusting the ratio between N1 and N2, themagnitude of the voltage V2 to be fed to the exciting circuit 110 canarbitrarily be set. As described above, the difference of the effectivevoltage of the AC power supply 106 can be adjusted by ON/OFF of theswitch SW and adjustment of the ratio between N1 and N2.

FIG. 3 is a time chart illustrating the voltage/current changingprocess. Time period from time t0 to time t1 (hereinafter, referred toas “first time period”) is a time period during which the switchingtransistor Q1 is ON while the switching transistor Q2 is OFF. Timeperiod from time t1 to time t2 (hereinafter, referred to as “second timeperiod”) is a time period during which the switching transistor Q1 isOFF while the switching transistor Q2 is ON. Time period from time t2 totime t3 (hereinafter, referred to as “third time period”) is a timeperiod during which the switching transistor Q1 is ON while theswitching transistor Q2 is OFF. Time period from time t3 to time t4(hereinafter, referred to as “fourth time period”) is a time periodduring which the switching transistor Q1 is OFF while the switchingtransistor Q2 is ON.

When the gate-source voltage VGS1 of the switching transistor Q1 exceedsa predetermined threshold, the switching transistor Q1 is in a saturatedstate. Thus, when the switching transistor Q1 is turned ON (conductive)at time t0 which is the start timing of the first time period, thesource-drain current IDS1 starts flowing. In other words, the current ISstarts flowing in the positive direction (the first current path). Atthis time, positive direction voltage V1 is applied to both ends of thetransformer T2 primary coil Lb. Further, positive direction voltage V2is applied to both ends of the transformer T2 secondary coil Li SinceN1>N2, V1>V2 is established.

Currents having the same phase start flowing in the exciting circuit110, feeding coil circuit 120, receiving coil circuit 130, and loadingcircuit 140.

When the switching transistor Q1 is turned OFF (non-conductive) at timet1 which is the start timing of the second time period, the source-draincurrent IDS1 does not flow. On the other hand, the switching transistorQ2 is turned ON (conductive), the source-drain current IDS2 startsflowing. That is, the current IS starts flowing in the negativedirection (the second current path). At this time, negative directionvoltage V1 is applied to both ends of the transformer T2 primary coilLb. Further, negative direction voltage V2 is applied to both ends ofthe transformer T2 secondary coil Li. After the third and fourth timeperiods, the same waveform as in the first and second time periodsrepeats.

FIG. 4 is a graph illustrating a relationship between the number ofwindings of the secondary coil in the coupling transformer T2 andsecondary voltage V2. The vertical axis represents input voltage Vin(V), and horizontal axis represents the number N2 of windings of thetransformer T2 secondary coil Li. The graph of FIG. 4 is drawn assuminga case where AC power of 20 (W) is fed by wireless. Further, the numberN1 of windings of the transformer T2 primary coil Lb is assumed to be23.

When the input voltage Vin is 282 (V), N2 is 4. As described above, whenthe switch SW is turned ON at the time when the effective voltage of theAC power supply 106 is 100 (V), the input voltage Vin is 282 (V). Inthis case, by setting N2 to 4, AC power of 20 (W) can be fed. When theeffective voltage of the AC power supply 106 is 200 (V), the switch SWis turned OFF and N2 is set to 4.

When the input voltage Vin is 141 (V), N2 is 8. When the switch SW isturned OFF at the time when the effective voltage of the AC power supply106 is 100 (V) and N2 is set to 8, AC power of 20 (W) can be fed. Whenthe input voltage Vin is 161 (V), N2 is set to 7.

FIG. 5 is a system configuration view of a wireless power transmissionsystem 100 which is a modification of the present embodiment. In thewireless power transmission system 100 of the modification, the powertransmission control circuit 200 directly drives the feeding coilcircuit 120 without intervention of the exciting circuit 110. Componentsdesignated by the same reference numerals as those of FIG. 2 have thesame or corresponding functions as those in FIG. 2.

The feeding coil circuit 120 in the modification is a circuit in whichthe transformer T2 secondary coil Li is connected in series to thefeeding coil L2 and capacitor C2. The transformer T2 secondary coil Liconstitutes a coupling transformer T2 together with the transformer T2primary coil Lb and receives AC power from the power transmissioncontrol circuit 200 by electromagnetic induction. Thus, the AC power maybe directly fed from the power transmission control circuit 200 to thefeeding coil circuit 120 without intervention of the exciting circuit110.

Second Embodiment

FIG. 6 is a view illustrating operation principle of the wireless powertransmission system 100 according to a second embodiment. As in the caseof the first embodiment, the wireless power transmission system 100according to the second embodiment includes the wireless power feeder116 and wireless power receiver 118. However, although the wirelesspower receiver 118 includes the power receiving LC resonance circuit302, the wireless power feeder 116 does not include the power feeding LCresonance circuit 300. That is, the power feeding coil L2 does notconstitute a part of the LC resonance circuit. More specifically, thepower feeding coil L2 does not form any resonance circuit with othercircuit elements included in the wireless power feeder 116. No capacitoris connected in series or in parallel to the power feeding coil L2.Thus, the power feeding coil L2 does not resonate in a frequency atwhich power transmission is performed.

The power feeding source VG supplies AC current of the resonancefrequency fr1 to the power feeding coil L2. The power feeding coil L2does not resonate but generates an AC magnetic field of the resonancefrequency fr1. The power receiving LC resonance circuit 302 resonates byreceiving the AC magnetic field. As a result, large AC current flows inthe power receiving LC resonance circuit 302. Studies conducted by thepresent inventor have revealed that formation of the LC resonancecircuit is not essential in the wireless power feeder 116. The powerfeeding coil L2 does not constitute apart of the power feeding LCresonance circuit, so that the wireless power feeder 116 does notresonate at the resonance frequency fr1. It has been generally believedthat, in the wireless power feeding of a magnetic field resonance type,making resonance circuits which are formed on both the power feedingside and power receiving side resonate at the same resonance frequencyfr1 (=fr0) allows power feeding of large power. However, it is foundthat even in the case where the wireless power feeder 116 does notcontain the power feeding LC resonance circuit 300, if the wirelesspower receiver 118 includes the power receiving LC resonance circuit302, the wireless power feeding of a magnetic field resonance type canbe achieved.

Even when the power feeding coil L2 and power receiving coil L3 aremagnetic-field-coupled to each other, a new resonance circuit (newresonance circuit formed by coupling of resonance circuits) is notformed due to absence of the capacitor C2. In this case, the strongerthe magnetic field coupling between the power feeding coil L2 and powerreceiving coil L3, the greater the influence exerted on the resonancefrequency of the power receiving LC resonance circuit 302. By supplyingAC current of this resonance frequency, that is, a frequency near theresonance frequency fr1 to the power feeding coil L2, the wireless powerfeeding of a magnetic field resonance type can be achieved. In thisconfiguration, the capacitor C2 need not be provided, which isadvantageous in terms of size and cost.

FIG. 7 is a system configuration view of the wireless power transmissionsystem 100 according to the second embodiment. In the wireless powertransmission system 100 of the second embodiment, the capacitor C2 isomitted. Other points are the same as the first embodiment.

The wireless power transmission system 100 has been described based onthe preferred embodiment. Since only the transformer T2 primary coil Lbis connected in series to the switching transistor Q1 in the firstcurrent path, VA (=Vin/2)=VDS1+V1 is established. When V1 issufficiently higher than VDS1, it is possible to prevent high voltagefrom being applied to the switching transistor Q1 even if the inputvoltage Vin is high. The same can be said for the second current path.

In the case of Patent Document 2, the primary coil L1 (corresponding totransformer T2 primary coil Lb) and capacitor C1 are made to resonate inseries, so that the impedance of the series circuit part is reduced,which may result in application of high voltage to the MOSFET Q1 and thelike. On the other hand, in the power transmission control circuit 200,it is not necessary to make the transformer T2 primary coil Lb resonate,so that the switching transistors Q1 and Q2 can easily be protected bythe inductance of the transformer T2 primary coil Lb.

In the power transmission control circuit 200, the AC current IR of theAC power supply 106 is rectified by the rectification circuit 108, andthe capacitors C5 and C6 are charged respectively. The chargedcapacitors C5 and C6 each function as a DC voltage source. Thus, whilethe capacitors C5 and C6 are charged by the AC power supply 106 in thepower supply section 102, the AC current IS of the resonance frequencyfr1 is generated by the oscillator 202 in the control section 104,whereby the AC power can be fed while utilizing the high voltage of theAC power supply 106. This eliminates need to use e.g., an AC adapter forreduction of the input voltage in the power supply section 102.

If an AC adapter or the like is provided in the power supply section102, loss of AC adapter occurs. In the wireless power feeder 116 in thepresent embodiment, a commercial power supply can be utilized as the ACpower supply 106 without modification, so that high power transmissionefficiency can be achieved. Further, the input voltage Vin can beadjusted by means of the switch SW. Furthermore, it is possible toadjust voltage to be fed to the exciting circuit 110 by adjusting theratio of the number of windings of the coupling transformer T2. Thus, asystem configuration that conforms to various power supply voltages ofthe commercial power supply can be achieved.

The above embodiments are merely illustrative of the present inventionand it will be appreciated by those skilled in the art that variousmodifications may be made to the components of the present invention anda combination of processing processes and that the modifications areincluded in the present invention.

The “AC power” used in the wireless power transmission system 100 may betransmitted not only as an energy but also as a signal. Even in the casewhere an analog signal or digital signal is fed by wireless, thewireless power transmission method of the present invention may beapplied.

1. A wireless power feeder for feeding power from a feeding coil to areceiving coil by wireless using a magnetic field resonance phenomenonbetween the feeding coil and receiving coil, comprising: a powertransmission control circuit that includes first and second currentpaths and makes first and second switches connected in seriesrespectively to the first and second current paths alternatelyconductive to feed AC power to a first coil included in both the firstand second current paths; the feeding coil; and an exciting circuit inwhich a second coil magnetically coupled to the first coil and anexciting coil magnetically coupled to the feeding coil are connected,wherein the exciting circuit receives AC power from the powertransmission control circuit through the second coil and feeds the ACpower to the feeding coil through the exciting coil.
 2. A wireless powerfeeder for feeding power from a feeding coil to a receiving coil bywireless using a magnetic field resonance phenomenon between the feedingcoil and receiving coil, comprising: a power transmission controlcircuit that includes first and second current paths and makes first andsecond switches connected in series respectively to the first and secondcurrent paths alternately conductive to feed AC power to a first coilincluded in both the first and second current paths; and a feeding coilcircuit in which the feeding coil and a second coil are connected,wherein the feeding coil circuit receives AC power from the powertransmission control circuit through the second coil magneticallycoupled to the first coil.
 3. The wireless power feeder according toclaim 2, wherein the power transmission control circuit includes: abridge rectification circuit in which diodes are bridge-connected atfour first to fourth connection points; an AC power supply connectedbetween the first and second connection points of the bridgerectification circuit; and a capacitor connected between the third andfourth connection points of the bridge rectification circuit and chargedby the AC power supply, wherein the power transmission control circuitfeeds AC power to the first coil using the capacitor as a DC voltagesource.
 4. The wireless power feeder according to claim 3, wherein twocapacitors are connected in series between the third and fourthconnection points, and the midpoint of the capacitors is connected toone of the first and second connection points.
 5. The wireless powerfeeder according to claim 4, comprising a switch for controllingelectrical conduction of a path extending from the midpoint to one ofthe first and second connection points.
 6. The wireless power feederaccording to claim 2, wherein the number of windings of the first coilis greater than that of the second coil.
 7. The wireless power feederaccording to claim 2, wherein input voltage of the power transmissioncontrol circuit is applied to one of a series circuit composed of thefirst coil and the first switch and a series circuit composed of thefirst coil and the second switch.
 8. The wireless power feeder accordingto claim 2, wherein the power transmission control circuit makes thefeeding coil that does not substantially resonate with circuit elementson the power feeding side feed the AC power to the receiving coil. 9.The wireless power feeder according to claim 2, wherein the powerfeeding coil does not form, together with the circuit elements on thepower feeding side, a resonance circuit having a resonance pointcorresponding to the resonance frequency of the receiving coil.
 10. Thewireless power feeder according to claim 2, wherein no capacitor isinserted in series or in parallel to the power feeding coil.
 11. Thewireless power feeder according to claim 2, wherein the feeding coil isconnected to a capacitor and form a circuit which resonates at aresonance frequency of the receiving coil.
 12. A wireless powertransmission system comprising: a wireless power feeder as claimed inclaims 2; the receiving coil; and a loading coil magnetically coupled tothe receiving coil and receives power that the receiving coil hasreceived from the feeding coil.
 13. The wireless power transmissionsystem according to claim 12, wherein the receiving coil is connected toa capacitor and form a circuit which resonates at a resonance frequencyof the feeding coil circuit.